Innate antiviral immunity comprises a host defense mechanism for restricting virus spread. The host cells recognizes invading viruses by specialized receptors known as pattern recognition receptors (PRRs) that can specifically identify pathogen associated molecular patterns (PAMPs). Toll-like receptors (TLRs) and RIG like helicase (RLH) receptors (e.g. RIGI and Mda5) are known to recognize virus specific PAMPs for induction of antiviral response mediated by activation of the transcription factors interferon regulatory factor-3 (IRF3). IRF3 activation results in production of antiviral cytokine interferon-a/ (IFN). Recently we identified nucleotide- binding oligomerization domain-2 (NOD2) protein, belonging to the nucleotide binding domain (NBD) and leucine-rich-region (LRR) containing family (known as NLRs) of cytoplasmic receptors as a viral PRR. Our preliminary studies have demonstrated that NOD2 can trigger an antiviral response (via activation of IRF3/IFN) following infection with human respiratory syncytial virus (RSV possess single stranded RNA or ssRNA as its genome), the major etiological viral agent causing worldwide pulmonary infections among infants, children and elderly. Therefore the current proposal is aimed at - a) establishing NOD2 as a new viral PRR, b) studying the mechanism of NOD2 activation, c) identifying and characterizing the molecules that act downstream of activated NOD2, and d) investigating the in vivo role of NOD2 during RSV pathogenesis, RSV induced lung inflammation and development of lung disease. Our preliminary studies demonstrated that expression of NOD2 in RSV infected cells results in activation of IRF3/IFN. Further studies revealed that - a) NOD2 can recognize ssRNA to activate IRF3/IFN, and b) mitochondrial outer membrane residing protein IPS-1 (MAVS) may act downstream of NOD2 for activation of IRF3/IFN . Thus, we hypothesize that NOD2 is a new member of viral PRR that can recognize viral ssRNA genome to launch an antiviral response following activation of mitochondrial IPS-1 protein. The hypothesis will be tested by focusing on the following specific aims: Aim # 1. Study the role of NOD2 during antiviral response against RSV - The major question of this aim is - Does NOD2 constitute a viral PRR that can activate IRF3/IFN in infected cells? We will investigate whether NOD2 is involved in IRF3/IFN activation during RSV infection of normal primary human bronchial epithelial (NHBE) cells. The physiological relevance of NOD2 during host defense against RSV pathogenesis and RSV mediated lung inflammation/disease will also be established by using NOD2 knock-out (KO) mice. Aim # 2. Study the mechanism of NOD2 activation - The major question of this aim is - a) Does NOD2 recognizes ssRNA and interacts with mitochondrial localized IPS-1 to activate IRF3/IFN? This question will be addressed by - a) studying association of NOD2 with ssRNA (RSV genome and synthetic ssRNA) and IPS-1, b) examining NOD2 mediated IRF3/IFN activation in cells lacking IPS-1, and c) identifying the specific NOD2 domain(s) involved in ssRNA recognition and IPS-1 interaction.